This invention relates to forming molecularly oriented containers of thermoplastic material, and more particularly to improvements in a preform reheat process for forming containers especially of nitrile thermoplastic material where strength is developed and wall thickness variation minimized especially in the lower container body portions.
Systems for forming containers from preforms reheated to molding temperature and then expanded in a mold are known. If the preform at the time of reforming is at molecular orientation temperature which is usually just above the glass transition temperature zone for the material, the resulting stressed containers have improved impact and burst strength which makes it possible to achieve a significant reduction in weight for a given performance over that required when forming at higher molding temperatures. As also known, thermoplastic materials containing a major proportion of polymerized nitrile-group-containing monomer can be fabricated into oriented containers in this manner and, though usable for packaging a wide variety of products such as foods, pharmaceuticals, personal care, household and industrial compositions and the like, in view of their exceptional strength and barrier properties they are especially desirable for packaging pressurized contents such as carbonated liquids in the form of soft drink beverages and beer.
Preforms of these and similar materials, however, present problems in a reheat process in that the temperature range within which orientation can be developed is quite narrow, as typically exemplified by the modulus-temperature plot of FIG. 6 of U.S. No. 3,814,784, and accordingly reheat process parameters for such materials must be tightly controlled. Consequently, though possible to form oriented high nitrile containers via a preform reheat process, it is important in obtaining high yields with minimum usage of material to precisely control variables such as preform wall thickness and the temperature pattern in the walls at the time of blowing. In this last respect, heat programming is usually employed to locally influence the extent of stretch during container formation. Also, though desirable for control it is difficult and most likely impossible to accurately measure temperature through the thickness of the preform wall after reaching orientation temperature since surface deformation will occur if a probe is used and radiation techniques are only effective to provide surface measurements.
Regarding the manner of forming preforms for such a reheat process, injection molding is preferred to minimize excess wall thickness variation since the plastic is molded in a cavity delimited by two surfaces defining the inside and the outside of the molded part vis-a-vis blowing where the inside surface of the part is not formed to a cavity wall. However, in pumping relatively stiff high nitrile thermoplastic material into an injection mold, frozen strains will inherently develop on cooling. Such strains relieve during reheat resulting in shrinkage along the preform length which has to be dealt with since no way has yet been found to entirely avoid developing such strains in an elongated preform. More specifically, a system employing temperature programming during reheat typically results in a region of the preform exposed to a heat source at one temperature gradually approaching the desired level for such region and then, because of strain relaxation, retracting to a position where the same plastic which had been exposed to the first source is now before a source set at a different temperature. When preforms subject to such overlapping exposure are expanded in the mold substantial thickness variability results which in turn can lead to excessively thin or thick areas and the apparent need for more material in the container than is really necessary for the intended end use.